1. Field of the Invention
This invention relates to a zoom lens, and particularly is suitable for use in an optical apparatus such as a video camera, a digital camera or a camera for film.
2. Description of Related Art
When an object is to be photographed by an optical apparatus such as a still camera or a video camera, if the object is bright, a stop aperture (aperture diameter) is made small to thereby limit the quantity of light. When the stop aperture becomes too small, the quality of image is deteriorated under the influence of diffraction. Also, the depth of focus is increased and therefore, the imprinting of dust adhering to the surface of a lens, a filter, etc. onto a photosensitive surface also poses a problem. So, in order to prevent the stop aperture from becoming too small even if the object is bright, there has heretofore been practised a technique of not making the stop aperture smaller than a predetermined size, but bringing an ND filter into the stop aperture to thereby decrease the quantity of passing light.
For example, in Japanese Patent Application Laid-Open No. 2000-305018, in a four-unit zoom lens comprising, in succession from the object side, a stationary first lens unit of positive refractive power, a second lens unit having negative refractive power and movable on an optical axis and having zooming magnification variation operation, a stationary third lens unit having positive refractive power, and a fourth lens unit having positive refractive power and movable on the optical axis for focusing and the correction of image displacement resulting from the magnification variation operation, an ND filter is adapted to be inserted rearwardly of the third lens unit so that the stop aperture need not be made very small.
Also, when the ND filter is inserted into an optical path, in order to prevent light (object light) incident on an image pickup element such as a CCD sensor from being reflected by the surface of the image pickup element, and the reflected light from being reflected by the ND filter and being re-incident on the image pickup element, the ND filter is disposed while being inclined with respect to the optical axis.
Also, Japanese Patent Application Laid-Open No. 5-281591 discloses a stop device using two ND filters disposed so as not to be parallel to each other in order to eliminate ghost occurring when light is reflected by the surfaces of the two ND filters.
Specifically, the ND filters are mounted on two stop vanes so as to cover a portion of a cut-away for forming a stop aperture, and these two ND filters are disposed at an angle so as not to be parallel to each other. Thereby, a ray of light transmitted through one ND filter and reflected by the other ND filter, and further reflected by one ND filter and then transmitted through the other ND filter is prevented from being incident on the light receiving surface of an image pickup element so that a ghost image may not be imprinted on a screen.
A zoom lens of the so-called four-unit rear focus type having, in succession from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power and in which the second lens unit is moved to thereby effect magnification variation, and the fourth lens unit is moved to thereby correct any fluctuation of image field resulting from magnification variation operation and focusing is effected by the fourth lens unit is proposed, for example, in Japanese Patent Application Laid-Open No. 62-206516, Japanese Patent Application Laid-Open No. 62-24213 (corresponding U.S. Pat. No. 4,859,042), Japanese Patent Application Laid-Open No. 62-247316, Japanese Patent Application Laid-Open No. 4-43311 (corresponding U.S. Pat. No. 5,189,558), Japanese Patent Application Laid-Open No. 5-060974, Japanese Patent Application Laid-Open No. 7-270684 (corresponding U.S. Pat. No. 5,963,378), Japanese Patent Application Laid-Open No. 7-318804 (corresponding U.S. Pat. No. 5,963,378), Japanese Patent Application Laid-Open No. 11-305124 (corresponding U.S. Pat. No. 6,166,864), etc.
The present invention has as its object the provision of a zoom lens of novel construction in which light reflected by an image pickup surface can be effectively prevented from being reflected by a filter member such as an ND filter and being again incident on the image pickup surface.
A zoom lens system according to an aspect of the present invention is provided, in succession from the front (the object side) to the rear (the image side), with a first lens unit of positive power (the inverse number of a focal length), a second lens unit of negative power moved during zooming, a stop unit, a third lens unit of positive power and a fourth lens unit of positive power moved during zooming. The zoom lens is characterized in that the stop unit has a stop blade (vane) forming an aperture, and a filter member for attenuating the quantity of light passing through the aperture, and the filter member is disposed while being inclined with respect to the optical axis of the zoom lens.